Researchers in the United Kingdom have discovered the fossilized exoskeleton of the world’s biggest arthropod. These massive millipede-like animals were the length of a vehicle and most likely inhabited Earth between 359 million and 299 million years ago, during the Carboniferous Period.
Scientists were already aware of the enormous millipede forebears known as Arthropleura, but the finding of the new preserved exoskeleton piece reveals that these ancient invertebrates could grow to be far larger than previously thought. In 2018, researchers discovered the new 326 million-year-old Arthropleura fossil inside a broken block of sandstone on a beach in Northumberland, northeast England.
The exoskeleton portion measures 2.5 feet (75 centimeters) in length and 1.8 feet (55 centimeters) in width. According to the researchers, the person that molted would have been around 8.5 feet (2.6 meters) long and weighed about 110 pounds (50 kilograms). “These would have been the largest animals on land in the Carboniferous,” said lead researcher Neil Davies, a geologist at the University of Cambridge in England’s Department of Earth Sciences.
Researchers had assumed that Arthropleura may potentially develop to these enormous levels, but they were nevertheless startled to see any concrete proof of this, he noted. The fossil discovery was “extremely fortunate,” according to Davies because the location where the exoskeleton was discovered “is not known for fossils.”
He also mentioned how lucky it was that the fossil was visible. “The fossil-containing rock had recently fallen off the cliff and cracked in just the precise location,” Davies explained. A former Ph.D. student who chance to be strolling by discovered the exposed fossil.
Molted exoskeletons do not often fossilize effectively since they degrade fast. However, this one had been unusually well kept. “It appears to have filled with sand shortly after it molted,” Davies explained. “It’s in a fossilized river channel, so it probably fell into a tiny river and was swiftly entombed in other silt.”
According to Davies, only two more Arthropleura fossils have ever been identified, both in Germany. The new fossil is the oldest and biggest found so far. Everything else experts know about the huge invertebrates comes from fossilized footsteps, or trackways, that they left behind in Europe and North America.
Based on earlier discoveries of fossils and trackways, the researchers were able to estimate the size of this new individual. “The width: length ratio of smaller Arthopleura samples is 4.78,” Davies noted. “So, because our animal was absolutely 55 cm wide, it’s 2.63 m long.”
Researchers are unsure what Arthropleura ate because no head has ever been discovered. However, they believe that these beasts were most likely vegetarians who ate trees, plants, and nuts. They may have consumed other tiny invertebrates as well.
It is also unknown how many legs Arthropleura possessed. “The more complete ones are considered to have 32 segments, although it is uncertain whether they had two legs every section 64 legs or 32 legs per two segments,” Davies added.
This individual’s trackways indicate that it had at least 20 legs, he noted. A new species of live millipede with 1,300 legs was discovered recently, according to Live Science, however, most existing species have less than 100 legs. Arthropleura would have been “quite widespread near the equator,” which at the time would have been significantly closer to what is now the United Kingdom, according to Davies.
The equator of Earth can migrate owing to a process known as true polar wander, which happens when a planet’s or moon’s outer layer travels around its core, tilting the crust relative to the object’s axis. According to Live Science, this “cosmic yo-yo” last occurred roughly 84 million years ago.
During the Carboniferous period, the tropical environment in what is now the United Kingdom, as well as a dearth of predators and other large creatures, likely enabled these invertebrates to develop to such amazing sizes. “It was probably simply a vast setting that suited them,” Davies added.
They would have enjoyed “a plentiful supply of food from trees and plants, with no competition from other animals.” However, conditions did not remain ideal for Arthropleura, and they became extinct around 45 million years after they first formed.
Davies believes that a movement in the equator to its current location during the early Permian Period, 299 million to 252 million years ago, modified the environment and allowed the earliest reptiles to survive on land. “They Arthropleura would have faced increased competition with fewer resources, eventually losing out to more efficient species,” he added.